The present invention may be more easily understood in the context of imaging arrays such as those used in digital photography to record an image. For the purposes of this discussion, an image will be defined as a two-dimensional array of digital values that represent the amount of light received during an exposure period at each pixel on a two-dimensional surface. It will be assumed that each pixel is a small rectangular area on that surface. In digital photography, the image is recorded by an imaging array in which each pixel includes a photodetector that measures the amount of light that falls on some portion of the pixel area. The smallest light exposure that can be measured is determined by the noise level in the pixels.
There is also a limit on the highest level of light that can be measured by a pixel or the imaging array as a whole. The high-level exposure limit can result from a number of factors. One of the high exposure limits results from the saturation of the storage capability of the photodiodes. The pixels store charge during the exposure period in the photodiodes. This charge is then readout via a readout circuit in the pixels to an analog-to-digital converter (ADC) that converts a signal related to the charge to a digital value. There is a limit on the amount of charge that can be stored in the photodiodes. In addition, the readout circuitry also has a limit on the maximum charge signal that can be converted to an analog signal before the readout circuit saturates. While some of these limitations can be overcome by utilizing more expensive imaging arrays, there is a limit on the maximum signal that can be accommodated. In addition, the increased cost of extending the high exposure limit presents problems to low cost imaging systems such as those used in inexpensive handheld devices such as cellular telephones and PDAs. For the purposes of this discussion, the dynamic range of an imaging array will be defined to be the ratio of the maximum signal for a pixel to the minimum signal that is above the noise.
An image that is to be captured by a camera can also be characterized by a dynamic range. In general, an image has a range of light intensities with a maximum intensity and a minimum intensity. In general, there is a minimum difference in light intensity between two pixels that needs to be distinguished in the final image. If the image is to capture the low light levels and the highest light levels with this precision, the required dynamic range for the image is the maximum image intensity divided by the minimum difference in pixel intensities.
The image dynamic range can be much greater than that of the camera for many applications. Consider an image having an intensity variation of 1000 to 1 within the image, i.e., an image dynamic range of 1000. If the camera uses an imaging array that has a dynamic range of 256 the corresponding 256 intensity levels will not cover the 1000 image intensity values without losing the image detail. If the available bits are used to capture the brightest information, then all of the pixel values below some level will be replaced by 0, and hence, the intensity information will be lost. Similarly if all of the available bits are used to capture the low light pixel information, all of the pixels above some level will be replaced by 255, and hence, those portions of the image will appear “washed out”
One solution to this mismatch problem involves using an imaging array in which the relationship between the output signal from each pixel and the light level during the exposure is non-linear. For example, photodetectors that generate a signal that is proportional to the logarithm of the light intensity have been proposed for applications in which the range of intensities is larger than the range that can be accommodated in simple photodiodes. If the ADC in a camera utilizing pixels based on such photodiodes has sufficient bits to capture the data, then the dynamic range of the camera could, in principle, be increased to a level that could accommodate higher dynamic range images.
Unfortunately, such an approach would substantially increase the cost of the camera and reduce the fill factor in each of the pixels. The additional circuitry needed to provide the logarithmic output in each pixel is significantly larger than the readout circuitry in conventional camera pixels. Hence, the area occupied by each pixel in the imaging array would be substantially larger, and the fraction of the pixel area that actually measures the light would decrease substantially. The cost of the imaging array increases with the area of silicon needed to construct the array, and hence, this added area leads to an increase in the cost of the imaging array.
The fill factor is the ratio of the photosensitive area of the pixel to the total area of the pixel. When the pixel fill factor is reduced, the camera sensitivity is decreased, which will reduce the camera dynamic range under low light conditions. Further, the charge holding capacity of the photodiode decreases with the fill factor, which reduces the camera dynamic range at high light conditions.
In addition, logarithmic pixel values cannot be directly displayed as an image in a manner that is desirable for a casual camera user on a low cost camera. The logarithmic scale distorts the image at all intensity levels, and hence, does not conform to the image seen by the camera user in any range of intensities.